1. Field of the Invention
The present invention relates to organic electrolytic solutions and to lithium batteries.
2. Description of the Related Art
As portable electronic devices such as camcorders, cellular phones, notebook PCs, etc. become lighter and more functional, considerable research is being conducted into batteries used as driving sources for the devices. Rechargeable lithium secondary batteries have energy densities per unit weight about three times greater than conventional lead batteries, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, etc. In addition, lithium rechargeable batteries can be rapidly charged. Accordingly, much research is directed at lithium rechargeable batteries.
Conventional aqueous electrolytic solutions are not usually suitable for lithium batteries because lithium anodes react vigorously with aqueous electrolytic solutions at high operation voltages. For this reason, organic electrolytic solutions prepared by dissolving lithium salts in organic solvents are used in lithium batteries. A good organic solvent has high ion conductivity, a high dielectric constant and low viscosity. However, it is very difficult to obtain a single solvent having all of these characteristics. As a result, mixed solvent systems have been used in lithium batteries. One such system includes an organic solvent having a high dielectric constant and an organic solvent having a low dielectric constant. Another system includes an organic solvent having a high dielectric constant and an organic solvent having low viscosity.
When polar, non-aqueous carbonate solvents are used in lithium secondary batteries, the carbon in the anodes reacts with the electrolytic solutions at initial charging and excess charge is used. Due to this irreversible reaction, passivation layers, such as solid electrolyte interface (SEI) films, are formed on anode surfaces. The SEI film prevents further decomposition of the electrolytic solution and stabilizes charging/discharging. Further, the SEI film acts as an ion channel, allowing only lithium ions to pass, and prevents organic solvents (which solvate lithium ions) flowing with the lithium ions from being cointercalated into the carbon anode. This prevents a collapse of the anode structure.
However, since high voltages of 4 V or greater are repeatedly applied over the SEI film consisting of only polar solvents and the lithium salt, it is difficult for the SEI film to retain the above functions. Cracks generate in the SEI film and the solvent continues to undergo the reduction reaction. As a result, insoluble salts precipitate on the inner and outer portions of the anode, and gas generates to form cracks in the anode structure, thereby reducing electronic connections. Thus, the internal resistance of the anode increases, thereby reducing the capacity of the battery. Further, due to the decomposition of the solvent, the amount of electrolytes decreases and the electrolytes are exhausted in the battery. Thus, a sufficient amount of the ions cannot be easily transferred. To address these problems, various compounds have been suggested for making the SEI film denser and stronger.